1. Field of the Invention
The present invention generally relates to valve driving apparatuses used for an internal combustion engine, and more particularly to a valve driving apparatus having an electromagnetic linear actuator which drives a valve body by generating a magnetic force corresponding to a current supplied thereto.
2. Description of the Related Art
An electromagnetically driven valve apparatus is known in which a valve body of a suction or exhaust valve of an internal combustion engine is driven by an electromagnetic attraction force generated by a solenoid. Such a valve apparatus can eliminate the need for a cam mechanism for driving a valve body which had been generally used in the prior art. Additionally, an opening and closing timing of the valve body can be arbitrarily changed, and thus an ideal opening and closing timing determined in response to operating conditions of the internal combustion engine can be realized.
In order to ensure good responsiveness of the electromagnetically driven valve apparatus, it is effective to generate a high-intensity magnetic field by supplying a large current to the solenoid.
On the other hand, an attracting force exerted on the valve body by the magnetic field generated by the solenoid is not only dependent on the intensity of the magnetic field, but is also dependent on a position of the valve body relative to the solenoid. That is, a greater attracting force is exerted on the valve body as the valve body moves inside the solenoid due to reluctance of the magnetic circuit formed around the valve body is decreased.
Accordingly, the generation of only a small magnetic field is sufficient to maintain the valve body at the end of the stroke of the valve body. That is, supplying relatively small current to the solenoid is sufficient to maintain the valve body at the end of the stroke. Therefore, in order to reduce power consumption, it is preferable not to supply a large current to the solenoid.
Japanese Patent Application No. 5-318672 suggests a valve apparatus in which a large current is supplied to a solenoid during an initial stage of a stroke of a valve body and excessive current is suppressed at the end of the stroke. This valve apparatus uses a discharge of a capacitor which is charged by a high voltage generated by means of a DC-DC converter. The high voltage generated by the DC-DC converter is supplied to the solenoid at an initial stage of the stroke which condition enables the valve body to start with good responsiveness. Thereafter, as the valve body moves inside the solenoid, current discharged from the capacitor decreases, and thus power saving can be achieved.
However, in the above-mentioned valve apparatus, the high voltage used for obtaining good responsiveness of the valve is generated by an expensive DC-DC converter. This increases the manufacturing cost of the valve apparatus.